The process of fabricating a semiconductor device includes the step of forming contact holes by dry etching using plasma in an interlayer insulating film (an insulating film containing silicon oxide as a main component) formed on the main surface of the wafer and filling a semiconductor or metal in the contact holes. In forming the contact holes, it is indispensable for an improved yield of the semiconductor device to fully open without any etch stop before exposure of the surface of the base semiconductor region or the underlying wiring. In view of the ever decreasing size of the contact hole and the resulting increase in the difficulty of etching, it is very important for executing a desired etching process to grasp the progress of the etching process or especially the etching depth accurately and reflect it in the processing conditions.
The situation in which the etching to form the contact holes is stopped midway and the underlying semiconductor region or the base wiring is not exposed is called an opening failure. In the prior art, in order to suppress the yield reduction due to the opening failure, it has been the practice to specify the cause of a defect by observing the cross section under SEM (scanning electron microscope) or inspecting the opening failure by the potential contrast method.
In the conventional method, however, a sample for the inspection device such as SEM is required to be prepared by actually sampling out a wafer from the lot. This requires a non-product wafer on the one hand and consumes the time of feedback to the fabrication process on the other hand, thereby reducing the productivity. Incidentally, the non-product wafer is defined as a wafer not directly contributing to the fabrication of a semiconductor device.
Now that the hole diameter has been decreased to almost less than 100 nm, the light in the wavelength range of ultraviolet to visible light hardly enters the pattern bottom without the effect of the pattern boundary, and the interference waveform measurement method using the light path length difference between the upper part and the bottom of the pattern cannot acquire a sufficiently practicable signal-to noise ratio (S/N).
As disclosed in JP-A-2000-131028 and JP-A-2001-284323, a means available to monitor the etching depth of the contact hole in real time is a method to determine the etching depth from the interference waveform due to the difference of the light path length between the upper part and the bottom of the pattern.